1. Field of the Invention
The present invention relates to a noise figure measuring system for efficiently measuring the noise figure of an optical amplifier for amplifying wavelength division multiplexed (WDM) signal light over the entire operational wavelength band of the optical amplifier.
2. Description of the Related Art
As a method of measuring the noise figure of an optical amplifier for amplifying WDM signal light, some noise figure measuring methods are known. Typical noise figure measuring methods include a pulse method, a probe method, and an interpolation method. FIG. 1 is a block diagram showing the pulse method. A plurality of optical signals having different wavelengths are output from a plurality of light sources 2#1 to 2#n (n is the number of wavelengths for measurement), respectively. The optical signals output from the light sources 2#1 to 2#n are wavelength division multiplexed by a multiplexer 4 to obtain WDM signal light. The WDM signal light output from the multiplexer 4 is input through a first optical switch 6 into an optical amplifier 8 whose noise figure is to be measured. Output light from the optical amplifier 8 is input through a second optical switch 10 into an optical spectrum analyzer 12. In the optical spectrum analyzer 12, signal light or amplified spontaneous emission light (ASE light) is measured.
FIG. 2A shows the operation of the first and second switches 6 and 10 in measuring the signal light. As shown in FIG. 2A, the first and second optical switches 6 and 10 are simultaneously turned on and off to measure the level of the signal light in the optical spectrum analyzer 12. FIG. 2B shows the operation of the first and second optical switches 6 and 10 in measuring the ASE light. As shown in FIG. 2B, the first and second optical switches 6 and 10 are operated in such a manner that when the first optical switch 6 is turned on, the second optical switch 10 is turned off, whereas when the first optical switch 6 is turned off, the second optical switch 10 is turned on, thereby measuring the level of the ASE light in the optical spectrum analyzer 12. That is, when the input light to the optical amplifier 8 is cut off, the level of the ASE light is measured to obtain the noise figure of the optical amplifier 8.
In the pulse method, it is required to prepare many light sources covering a desired wavelength band where the noise figure is to be measured. Further, special high-speed optical switches are required. Accordingly, the scale of a measuring system becomes larger with an increase in the number of multiplexed wavelengths, thus causing an increase in cost.
FIG. 3A is a block diagram showing the probe method. The probe method employs several light sources 2#1 to 2#m for stabilizing the characteristics of an optical amplifier 8 whose noise figure is to be measured and a probe light source 14 for measurement of the noise figure. Optical signals from the light sources 2#1 to 2#m and probe light from the probe light source 14 are wavelength division multiplexed by a multiplexer 4 to obtain WDM signal light. The WDM signal light is then input into the optical amplifier 8. Output light from the optical amplifier 8 is measured by an optical spectrum analyzer 12. FIG. 3B illustrates the operation of the probe method. Several optical signals 3 for stabilizing the characteristics of the optical amplifier 8 are spaced a given wavelength, and probe light 15 output from the probe light source 14 operates to sweep each optical signal 3 as shown by an arrow 16, thereby performing the measurement over the entire wavelength band to obtain the noise figure of the optical amplifier 8.
In the probe method, several light sources and one probe light source are merely required irrespective of the number of multiplexed wavelengths, so that the scale of a measuring system is not enlarged with an increase in the number of multiplexed wavelengths. However, the measurement at one time is performed for the characteristics at one wavelength, so that the measurement time increases with an increase in the number of wavelengths for measurement.
FIG. 4A is a block diagram showing the interpolation method. A plurality of optical signals output from a plurality of light sources 2#1 to 2#n are wavelength division multiplexed by a multiplexer 4 to obtain WDM signal light. The WDM signal light from the multiplexer 4 is input into an optical amplifier 8 whose noise figure is to be measured. Output light from the optical amplifier 8 is measured by an optical spectrum analyzer 12. FIG. 4B illustrates the operation of the interpolation method. As shown in FIG. 4B, ASE light 17 is removed from the spectrum of the output light to estimate the level of ASE light included in the signal light 3, thereby obtaining the noise figure of the optical amplifier 8.
Like the pulse method, the interpolation method is also required to prepare many light sources covering a desired wavelength band where the noise figure is to be measured, so that the scale of a measuring system becomes larger with an increase in the number of multiplexed wavelengths. Further, the estimation of the ASE level becomes more difficult with a decrease in wavelength spacing, causing an increase in measurement error.